Vertebrates achieve internal homeostasis during infection or injury by balancing the activities of proinflammatory and anti-inflammatory pathways. However, in many disease conditions, this internal homeostasis becomes out of balance. For example, endotoxin (lipopolysaccharide, LPS) produced by all Gram-negative bacteria activates macrophages to release cytokines that are potentially lethal (Tracey et al., 1986; Wang et al., 1999; Nathan, 1987; Dinarello, 1994).
Inflammation and other deleterious conditions (such as septic shock caused by endotoxin exposure) are often induced by proinflammatory cytokines, such as tumor necrosis factor (TNF; also known as TNFα or cachectin), interleukin (IL)-1α, IL-1β, IL-6, IL-8, IL-18, interferonγ, platelet-activating factor (PAF), macrophage migration inhibitory factor (MIF), and other compounds (Thompson, 1998). Certain other compounds, for example high mobility group protein 1 (HMG-1), are induced during various conditions such as sepsis and can also serve as proinflammatory cytokines (PCT publication WO 00/47104). These proinflammatory cytokines are produced by several different cell types, most importantly immune cells (for example monocytes, macrophages and neutrophils), but also non-immune cells such as fibroblasts, osteoblasts, smooth muscle cells, epithelial cells, and neurons (Zhang and Tracey, 1998). Proinflammatory cytokines contribute to various disorders, notably sepsis, through their release during an inflammatory cytokine cascade.
Mammals respond to inflammation caused by inflammatory cytokine cascades in part through central nervous system regulation. This response has been characterized in detail with respect to systemic humoral response mechanisms during inflammatory responses to endotoxin (Besedovsky et al., 1986; Woiciechowsky et al., 1998; Hu et al., 1991; Lipton and Catania, 1997). In one set of responses, afferent vagus nerve fibers are activated by endotoxin or cytokines, stimulating the release of humoral anti-inflammatory responses through glucocorticoid hormone release (Watkins and Maier, 1999; Sternberg, 1997; Scheinman et al., 1995). Previous work elucidated a role for vagus nerve signaling as a critical component in the afferent loop that modulates the adrenocorticotropin and fever responses to systemic endotoxemia and cytokinemia (Gaykema et al., 1995; Fleshner et al., 1998; Watkins et al., 1995; Romanovsky et al., 1997).
Another set of responses is through efferent vagus nerve signaling, termed the “cholinergic anti-inflammatory pathway” (Borovikova et al., 2000). Stimulation of the efferent vagus nerve attenuates systemic inflammatory responses and inhibits TNF release (Id.; Bernik et al., 2002; Tracey et al., 2001; U.S. patent application Ser. No. 09/855,446). Acetylcholine, the principle neurotransmitter of the vagus nerve, attenuates macrophage cytokine synthesis by signaling through α-bungarotoxin-sensitive nicotinic acetylcholine receptors, but the identity of the essential macrophage receptor is unknown.
Nicotinic acetylcholine receptors are a family of ligand-gated, pentameric ion channels. In humans, 16 different subunits (α1-7, α9-10, β1-4, δ, ε, and γ) have been identified that form a large number of homo- and hetero-pentameric receptors with distinct structural and pharmacological properties (Lindstrom, 1995; Leonard and Bertrand, 2001; Le Novere and Changeux, 1995). The main known function of this receptor family is to transmit signals for the neurotransmitter acetylcholine at neuromuscular junctions and in the central and peripheral nervous systems (Lindstrom, 1995; Leonard and Bertrand, 2001; Le Novere and Changeux, 1995, Marubio and Changeux, 2000; Steinlein, 1998). Our previous work indicated the presence of α-bungarotoxin-sensitive nicotinic receptors on primary human macrophages (Borovikova et al., 2000), but the identity of the specific receptor subunit was unknown.
Knowledge of the particular nicotinic receptor that is responsible for inhibiting inflammation would be useful to identify specific agonists of the receptor that would inhibit inflammation. Such agonists would be likely to have fewer side effects than currently identified agonists that are relatively non-specific. The identity of other physiological effects influenced by the anti-inflammatory receptor would also be facilitated.